Current avionics systems exhibit high levels of both design complexity and functional reliability. Complexity of design implies that factory or depot-level repair technicians require extensive technical training in order to maintain line replaceable units (LRUs) or shop repairable units (SRUs). High LRU/SRU reliability implies that frequent maintenance of the units isnot required and, consequently, technician considered to be "experts" on specific rather than ganeric types of units often exhibit a large amount of nonproductivity. The alternative for factory or depot-level maintenance is to use "nonexpert" technicians for maintaining several types of LRUs/SRUs. Since these technicians are not skilled in the repair of all types of units, both maintenance time and documentation requirements per LRU/SRU are greatly increased.
In a typical factory or depot-level repair procedure, units under test (UUTs) are functionally tested on an automatic test system (ATS) to identify failed operational modes. The UUT may be a single circuit card taken from a large system, or a functional unit comprising several circuit cards. The ATS comprises an automatic test equipment (ATE) controller and an associated group of test instruments such as digital multimeters, waveform generators, counters, timers, and power supplies, a set of relay switches, and a load board containing loads (e.g., resistors) required to test certain circuits. The ATE controller is a computer running under the control of a program generally referred to as a functional test procedure (FTP). Each FTP is written and utilized for a particular UUT. The FTP instructs the ATE controller which signals to apply, which tests to perform, and what the results of these tests should be under normal circumstances. The functional test procedure is used to determine if the unit is functionally sound. If any of the tests in the functional test procedure do not pass, then the results of the FTP test are forwarded, along with the UUT, to a maintenance technician for analysis and repair. The technician will analyze the FTP results, and use a variety of test equipment, possible including the ATS, to manually troubleshoot the bad UUT to find the fault. Once the faulty components have been found, the UUT is sent to the repair bench where the bad components are replaced. The UUT is then again sent to the ATS to be retested. If the FTP fails again, then the process is repeated until the functional test procedure passes.
The procedures used in such functional tests, usually in printed form, provide minimal information about fault isolation, and generally require the maintenance technician to subject the UUT to further diagnostic procedures. Development of diagnostic procedures is time consuming, and is usually performed after the design of the UUT has been completed. Verification of teh accuracy of a diagnostic procedure is generally performed by fault insertion, an aproach that typically requires a considerable amount of rework of the UUT, or the building of a special UUT for verification purposes. Efforts have been made to eliminate some of these problems, by coding diagnostic procedures directly into functional test procedures. However, this approach has generally been unproductive, because ATS test time is significantly increased, and because the amount of software required to perform the combined FTP and diagnostic test procedure is increased by a factor of three or more.
There have been some preliminary attempts to use artificial intelligence systems to facilitate the functional testing of electronic systems. In general, these efforts have taken two directions. One direction has been to use expert system techniques to automatically produce functional test procedures for current automatic test systems. A second direction has been to develop expert systems for use by repair technicians, to supplement the technician's knowledge, and to permit the technician to diagnose faults in UUTs for which the technician has not been fully trained. The commercial utility of these approaches has yet to be determined.